A conventional asynchronous data communication system operating in a start-stop synchronization mode initially converts parallel data bits into serial data bits which are then transferred, i.e. transmitted, from a transmitter to a receiver.
This system is described briefly with respect to an example in which a data value is transferred from a digitizer to a host computer. FIG. 10 is an overall perspective view of a communication system including a digitizer and a host computer. The digitizer 1 is comprised of a tablet planar sensor 2 which defines a two-dimensional coordinate plane. A suitable input tool such as a stylus pen 3 is used to designate a given point on a surface of the tablet planar sensor 2 so that the sensor 2 generates an analog detection signal indicative of the designated point. The digitizer 1 further includes a processing circuit unit operative to calculate data representative of a two-dimensional coordinate value of the designated point according to the analog detection signal. The calculated data are fed to the host computer 5 through a signal cable 4. The host computer 5 is provided with a plurality of different type interfaces for connection with peripheral input and output terminal devices. The interfaces are selected according to a form of the signal to be transferred.
FIG. 11 is a waveform of an output signal transmitted from the digitizer. For example, the digitizer generates a coordinate data value in the form of eight parallel data bits. This eight-bit parallel data value is subjected to parallel/serial conversion to produce an eight-bit serial data signal as shown in FIG. 11. For example, eight parallel data bits represented by 10110101 are converted into a corresponding eight-bit serial data signal represented by 1-0-1-1-0-1-0-1. Such mode of bit data transmission is called an asynchronous system of start-stop synchronization mode as noted before. In order to receive a data signal formed according to the asynchronous system of start-stop synchronization mode, the host computer is typically provided with a particular interface based on an RS-232C standard. Therefore, a peripheral terminal device is generally connected to the host computer through a general purpose RS-232C interface.
The RS-232C interface is normally shared by various kinds of peripheral terminal devices adopted to execute data transfer or transaction according to the asynchronous system of start-stop synchronization mode. However, the RS-232C interface can accept a limited number of peripheral terminal devices to be concurrently accessed by the host computer. Therefore, it is desired to provide a different communication mode for effectively executing data transfer or transaction through other interfaces of the host computer than the RS-232C interface.
The typical host computer is provided with a bus mouse interface specifically designed to be connected to a bus mouse device. A brief description is given herewith for the bus mouse for facilitating understanding of the present invention. A bus mouse is generally utilized as a coordinate input terminal device for a computer. However, the bus mouse can treat only a relative displacement value. Further, since the bus mouse has an overall size considerably greater than a stylus pen used as an input tool of the digitizer, the bus mouse cannot carry out character input and menu selection. On the other hand, the digitizer can treat the absolute coordinate value and input characters. Since the bus mouse has a relatively low price and utilizes a relatively simple interface construction, recently developed computers are generally compatible with a standard bus mouse. FIG. 12 is a schematic diagram of the construction of a typical bus mouse device. The bus mouse is manually moved to derive input computer signals representing the amount of its own displacement. The bus mouse is provided on its bottom portion with a ball member 6 to enable the movement of the body of the bus mouse. A pair of sensors 7 and 8 are disposed in contact with the ball member 6 to detect separately X-axis and Y-axis components of the displacement amount.
FIG. 13 is a schematic illustration for the operation of the bus mouse sensor of FIG. 12. The X-axis sensor 7 has a rotary shaft in contact with the ball member 6. Although not shown in the figure, Y-axis sensor 8 has another rotary shaft disposed orthogonally to the rotary shaft of the X-axis sensor 7, in contact with the ball member 6. As ball 6 rolls due to the displacement of the bus mouse, the rotary shaft of the X-axis sensor 7 is rotated according to the X-axis component of the displacement to produce an output signal having pulse trains XA and XB. Concurrently, the bus mouse outputs a switching signal S1 and S2 indicative of operating state or condition of the bus mouse.
FIG. 14 represents waveforms for output signals of the bus mouse. As shown in the figure, the output signal is composed of two pulse trains. When the bus mouse is displaced in the plus or positive direction in terms of X-axis, pulse train XA has a leading phase with respect to pulse train XB. On the other hand, when the bus mouse is displaced in the minus or negative direction in terms of X-axis, the pulse train XA has a lagging phase with respect to pulse train XB. By such operation, the amount of bus mouse displacement per unit time is detected in terms of a number of pulse edges per unit time contained in the output signal; the displacement direction is indicated by the relative phase condition of the output signal.
The typical host computer is provided with a bus mouse interface responsive to the output and switching signals from a bus mouse device. The bus mouse interface contains first and second eight-bit counters respectively corresponding to the X and Y coordinates for counting pulse edges of the X and Y output signals. These calculation results are retrieved from the respective eight-bit counters each period of a given interval of, e.g., 8 ms, regulated by an internal time in the host computer to detect the amount of bus mouse displacement and the switch condition of the bus mouse every period.
As shown in FIG. 11, a conventional digitizer produces an output signal composed of a binary bit series according to the asynchronous system of start-stop synchronization mode. On the other hand, as shown in FIG. 14, the bus mouse device produces an output signal composed of a two-phase pulse train. Therefore, the conventional digitizer has a different output signal format from that of the bus mouse device. Hence the digitizer cannot be directly connected to the bus mouse interface of the host computer.
As understood from the above description, the binary bit serial data value cannot be supplied to the bus mouse interface of the host computer, if it is based on the asynchronous system of start-stop synchronization mode. However, the bus mouse interface is so specialized and therefore is not as frequently occupied as the general RS-232C interface. Thus, it is desired in commercial use to carry out the data transfer or transactions by using a channel of the bus mouse interface. However, the conventional data communication system could not satisfy such a demand in the market.
In view of the above noted drawbacks of the conventional data communication system, an object of the present invention is to provide a new communication system for transferring parallel data bits from a transmitter, such as a peripheral terminal device, to a receiver, such as a host computer, through a bus mouse interface provided in the host computer.